1. Field of the Invention
The present invention relates to image processing, and more particularly, to Y/C separation.
2. Description of the Related Art
In many applications, image data are carried by a composite signal for transmission. Please refer to FIG. 1, which is a diagram of a frequency spectrum of a composite signal. As shown in FIG. 1, the composite signal comprises luminance information and chrominance information, wherein the chrominance information is superimposed to the high frequency part of the luminance information through modulation.
After receiving the composite signal, the video decoder needs to separately extract luminance information and chrominance information from the composite signal for follow-up processing. Such an operation is then termed as luminance/chrominance separation, or Y/C separation.
In general, the above-mentioned Y/C separation embodies a filtering operation upon the composite signal such that the chrominance information can be separated from the high-frequency part of the luminance information. However, different filtering bands often result in varied filtering results. For example, in a case where the filter has narrower filtering band (such as the frequency band W1 shown in FIG. 1), when the filtered image is displayed, the boundary of the color tends to be blurred because the entirety of the chrominance information cannot be fully extracted from the composite signal. On the other hand, in a case where the filter has a wider filtering band (such as the frequency band W2 shown in FIG. 1), although the chrominance information can be fully extracted from the composite signal, the filtered image signal, which is regarded as the chrominance signal, may actually comprise not only the genuine chrominance information, but also some of the high frequency component of the luminance information. This means that some high frequency part of the luminance information is incorrectly determined as chrominance information, and such phenomenon is called as cross-color effect. Therefore, when the image is displayed, although the color boundary tends to be sharper, the above-mentioned cross-color effect introduces infidelity to the image in such a way that the displaying performance of the image deteriorates.
As a result, no matter which filtering band (such as the frequency bands W1 and W2) is applied, there is always a corresponding problem to be solved.